New materials with high dielectric constants are needed in the electronics industry for applications at high frequencies (above about 500 MHz) to enable further miniaturization. These are particularly useful if they can be molded into flat sheets or thin films for such uses as circuit boards at microwave frequencies, high energy density capacitors, filters, antennas, buried components and multichip modules. A low loss tangent is also important to reduce noise and signal attenuation in high frequency circuits. These materials have a variety of potential end uses, especially in wireless communications. Certain ceramic materials have the desired high dielectric constant and low loss tangent, but they are not readily made into sheets. Ceramic materials are also brittle.
One approach to making sheets and other shaped components with the desired properties is to utilize a composition comprising a polymeric matrix and a ceramic filler with a high dielectric constant. Such compositions are often referred to as "composites," and are referred to herein as composites or high dielectric composites or compositions. A high level of ceramic filler is needed to make a composite that has a high dielectric constant. It is important that these highly filled composites be readily fabricated, as by injection molding. The composites must also be stable to changes in ambient moisture (humidity) and temperature. Resistance to elevated temperatures, high mechanical strength, impact resistance, and chemical resistance are also desirable. Finally, in many applications, sheets and films of these materials will need to be made into laminates with copper and/or other materials.
Numerous polymer composite dielectric materials are known. For example, U.S. Pat. Nos. 4,335,180 and 5,358,775, both of which are assigned to Rogers Corporation, teach composites of fluoropolymers, preferably poly-(tetrafluoroethylene) (PTFE), and ceramic materials for use as high dielectric materials. Rogers Corporation sells a product containing PTFE and a ceramic filler for use as a high dielectric film. It is however difficult to make thin films and other shaped composites of PTFE containing a filler.
U.S. Pat. No. 5,154,973 generally describes composites of thermoplastic polymers and ceramics having a high dielectric constant. German patent publication 3,242,657 describes composites of BaTiO.sub.3 in poly(ethylene terephthalate). Japanese patent publication 98,069 teaches BaTiO.sub.3 in poly(phenylene sulfide). Composites of high dielectric ceramics in thermosets are also known. For example, a high dielectric composite in which an epoxy resin based on bisphenol F epoxy is filled with barium titanate has been described (S. Asai, et al., IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. 16, No. 5, August, 1993, pp. 499-504). The composites based on epoxy are easy to process before the epoxy resins set because of the low viscosity of the epoxy prepolymer. These composites have dielectric constants up to about 20.
Compositions having high dielectric constants that utilize thermoplastic polymers and are easy to fabricate without complex processing are disclosed herein. Preferred compositions also have a low loss tangent.